Coal fly ash is known to contain a variety of metals such as aluminium, titanium, zinc, copper, cobalt and others, usually in the form of insoluble metal oxides. Many of these metals and their oxides are environmentally hazardous, in that when the coal fly ash is disposed in land fills, in the sea or in other waterways metal oxides are leached out and find their way into potable water and into animal and vegetable food for human consumption. For example, it has been reported that food sources contaminated with aluminium may affect the brain and trigger off Alzheimer disease.
It is thus evident that the disposal of coal fly ash which in many industrialized nations amount to millions of tons yearly, poses a serious environmental problem.
Depending on their nature and concentration, the metals present in coal fly ash as oxides may in themselves be of industrial value. Accordingly, the gravity of the environmental problem arising in connection with the disposal of coal fly ash on the one hand, and the fact on the other hand that the recovery of metals from such waste products may in itself be of economic value, both of which have long been released, have for a long time prompted investigators in the field to look for processes for the removal of metal oxides from coal fly ash.
By some proposals, metals are extracted from coal fly ash by conventional chemical leaching processes which, however, are not economically attractive as they require the use of strong acids and other chemicals and expensive acid-proof equipment. It has further been proposed in relation to coal fly ash to employ bioleaching methods, but the known microorganisms utilized for this purpose require extremely acidic conditions for growing and, moreover, also produce acids themselves so that here again similar processing problems arise. Moreover, in addition to being technologically onerous, these known chemical and biotechnological, acid-consuming processes are in themselves ecologically hazardous due to the need of handling and disposing large amounts of acids.
In principle, biotechnological leaching could provide a satisfactory and environment friendly solution to the problem of removing metal oxides from coal fly ash, provided it is operated under mild conditions. However, hitherto no suitable microorganism was found that would combine efficient extraction of metal oxides with environment friendly operating conditions.
It is the object of the present invention to provide an ecologically and economically viable process for the bioextraction of metal oxides from coal fly ash.
It is a further object of the present invention to provide microorganisms that can be used for that purpose.